The origin of reservoir fluids in the geothermal field

Applied Geochemistry 16 (2001) 1595–1610 www.elsevier.com/locate/apgeochem

The origin of reservoir fluids in the geothermal field of Los Azufres, Mexico — isotopical and hydrological indications Peter Birkle a,*, Broder Merkel b, Enrique Portugal a, Ignacio S. Torres-Alvarado c a Instituto de Investigaciones Elećtricas, Unidad de Geotermia, AP 1-475, Cuernavaca, Mor., 62001 Mexico Technical University of Freiberg, Institute of Geology, Gustav-Zeuner-Str. 12, 09596 Freiberg/Saxony, Germany c Centro de Investigacioń en Energıá, Universidad Nacional Autońoma de Me´xico, AP 34, Temixco, Mor., 62580 Mexico b

Received 30 November 1999; accepted 30 December 2000 Editorial handling by H. A´rmannsson

Abstract The calculation of hydrological balance resulted in a potential, average annual infiltration rate of 446 206 mm/m2 for the Los Azufres geothermal area, which corresponds to a total of 82106 m3 per a. Due to the highly fractured and faulted structure of the volcanic formations, a considerable potential for the infiltration of recent meteoric water into deeper sections of the volcanic formations can be assumed. Isotopic data indicate the minor importance of recent meteoric water for the recharge of the geothermal reservoir. Very negative 13C values can be explained by the input of organic C from the surface, but the lack of 14C in the deep fluids reflects a pre-historic age for the infiltration event of fossil meteoric water. The dilution of the meteoric water by 14C-free CO2 gas from a shallow magma chamber complicates the exact age determination of the infiltration event, which probably occurred during the Late Pleistocene or Early Holocene glacial period. Strong water–rock interaction processes, such as sericitization/chloritization, caused the primary brine composition to be camouflaged. A preliminary hydrological model of the reservoir can be postulated as follows: the fossil hydrodynamic system was characterized by the infiltration of meteoric water and mixing with andesitic and/or magmatic water. Strong water–rock interaction processes in the main part of the production zone prove the existence of former active fluid circulation systems. Due to changes in pressure and temperature, the rising fluids get separated into liquid and vapour phases at a depth of 1500 m. After cooling, the main portions of both phases remain within the convective reservoir cycle. Isotope analyses of hot spring waters indicate the direct communication of the reservoir with the surface at some local outcrops. A recent reactivation of the hydrodynamic system is caused by the geothermal production, as indicated by the detection of lateral communication between some production and reinjection wells. # 2001 Elsevier Science Ltd. All rights reserved.

1. Introduction Since the beginning of the 1980s, a mixture of geothermal fluid and vapour has been extracted from a depth between 350 and 2500 m in the Los Azufres geothermal field, located 220 km NW of Mexico City (Fig. 1). The origin of the fluids, the influence of surface recharge by meteoric water and/or regional aquifer * Corresponding author. Fax: +52-73-182526. E-mail address: [email protected] (P. Birkle).

systems, as well as the hydraulic conditions of the deep circulation systems are still unknown. These questions will be of importance for the prediction of the future potential of the geothermal production cycle. In the past, reservoir models mainly elaborated on exploratory aspects, applying thermodynamic parameters, such as P/ T-conditions (Iglesias and Arellano, 1985) or phase distributions (Nieva et al., 1983). Artificial tracer tests with KI (Iglesias, 1983) and 192Ir (Aragoń, 1985) were not able to detect communication systems between individual wells, although reinjected brines caused elevated

0883-2927/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved. PII: S0883-2927(01)00031-2

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P. Birkle et al. / Applied Geochemistry 16 (2001) 1595–1610

Fig. 1. The Los Azufres geothermal field is located 220 km NW of the capital Mexico City. Its northern part discharges towards the hydrological basin of the Cuitzeo Lake, whereas its southern part (Tejamaniles) discharges towards the River Balsas basin.

CO2 and N2 concentrations in adjacent production wells (Horne and Puente, 1989). This paper tries to improve the understanding of the hydrodynamic processes in the Los Azufres geothermal reservoir, which comprise potential circulation systems and recharge processes, as well as details of the type and origin of the fluids. Stable isotope analysis of surface outcrops and reservoir brines give indications about the

existence of vertical communication systems, as well as about the magnitude of water–rock interaction processes. The use of radioactive isotopes and the determination of their residence time reveal the existence of recent or fossil hydrodynamic processes within the reservoir. Hydrological balance calculations for the study area indicate the potential of recent infiltration into the shallow and deep aquifer systems.


2. Methods Monthly and annual climatological data from 14 stations (one inside the field, 13 in the vicinity) from 1955 to 1979 (Cedillo et al., 1981) was used to calculate the hydrological balance for the study area, such as the average and total precipitation (P) and the potential evaporation rate (ETpot) (Birkle, 1998). The actual evapotranspiration (ETactual), which represents the sum of transpiration by plants and evaporated precipitation water, was determined by semi-empirical equations. The methods of Turc (Gray, 1973), Morton (Morton, 1965), Budyko (Kuzmin and Vershinin, 1974), and Coutagne (Remendieras, 1974) are based on climatological, atmospheric and vegetation data, such as temperature, relative humidity, latent vapour heat, solar radiation, albedo and potential evaporation. During the dry season from October to May, the volume of surface discharge was derived by regular hydrometric measurements with flowmeter equipment. Due to the lack of installed hydrometric stations in the study area, significant runoff fluctuations during the wet season were not measured directly in the field. Wet season discharges from the Los Azufres area were derived by the analogue comparison of the dry/wet season discharge proportions from the lower course, external stations. A methodical error of 10%, 25% and 3% is accepted for the determination of P, ETactual, and the runoff measurements, respectively. Stable isotope analysis (D, 18O) of geothermal brines, and cold and hot springs were performed at the Unidad de Geotermia at the Instituto de Investigaciones Elećtricas, Cuernavaca, Mexico and the Technical University of Freiberg/Saxony, Germany. Four hundred liters of geothermal brine, with an HCO 3 concentration of 143.3 mg/l, were extracted from well Az-43. The carbonates were precipitated with BaCl2 as BaCO3. CO2 from 10 gas samples from the wells Az-2, 4, 5, 6, 9, 22, 26, 28, 38, and 41 was concentrated in a vacuum vessel with a NaOH-saturated solution. The sampling preparation for the determination of 13C from CO2 was performed according to Giggenbach and Goguel (1989). Tritium, 13C and 14C-analysis on surface samples and geothermal brines were carried out at the Institute for Applied Physics at the Technical University of Freiberg in Saxony, Germany. The residence times of the isotopes were calculated with MULTIS (Richter et al., 1993).

3. Geological setting The volcanic formations of Los Azufres have been described by several authors (Dobson and Mahood, 1985; Cathelineau et al., 1987). Geologically, two principal divisions can be distinguished.

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1. A silicic sequence of rhyodacites, rhyolites and dacites with ages between 1.0 and 0.15 My and thickness up to 1000 m (Dobson and Mahood, 1985). This sequence serves as a caprock to the aquifer from the surface, allowing the geothermal system to pressurise. 2. A 2700 m thick interstratification of lava flows and pyroclastic rocks, of andesitic to basaltic composition with ages between 18 and 1 Ma, forming the local basement (Dobson and Mahood, 1985). This unit provides the main aquifer with fluid flow through fractures and faults, sometimes reaching the surface. The thermal fluids are NaCl rich waters with high CO2 and H2S contents, and a pH between 5.8 and 7.2. Average Cl contents are 3100 mg/kg and CO2 may represent as much as 90% of the total gas phase. Fluid temperatures reach values as high as 320 C, but 240–280 C are normal in the field. Hydrothermal alteration has affected most rocks in the geothermal field to a varying extent. Studies of hydrothermal alteration at Los Azufres have been carried out, among others, by Cathelineau et al. (1985), and Torres-Alvarado (1996). These studies have shown that partial to complete hydrothermal metamorphism has occurred. The most important alteration assemblages with increasing depth are: argillitization/silicification, zeolite/ calcite formation, sericitization/chloritization, and chloritization/epidotization.

4. Results 4.1. Hydrological balance The area of the Los Azufres field forms two hydrological basins, mainly because of its isolated, topographical high plateau position (Fig. 1). The northern part (Marı´taro) discharges towards the flat plain of the Cuitzeo Lake basin in NW-direction, whereas the southern part of the field (Tejamaniles) discharges towards the river Balsas basin in S- and SE-directions. The detailed map of the geothermal field area (Fig. 2) shows the water division between both two lateral sections of the geothermal field. The radial structure of rivers and creeks with the Los Azufres mountain peak as the central core area confirms the unusual hydrological discharge conditions. Two case studies were developed to determine the size of the hydrological impact area of the geothermal reservoir. a. Geothermal field area: Hydrological processes, which affect the deep reservoir, are restricted to the lateral surface dimension of the geothermal field (total size: 30 km2) (Fig. 2).

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b. Extended study area: the entire mountain plateau, including the surrounding lower foothill plains influences the geothermal reservoir by means of infiltration processes (total size: 183 km2). The seasonal distribution of P, ETpot and ETactual values in the study area is described according to Fig. 3: from March to May, high ETpot rates and the lack of precipitation imply the complete absence of infiltration

processes. The same is probably true for the winter months December to February, when lower temperature conditions cause a decrease in ETpot. Major infiltration processes can be expected during the rainy season from June to September, when P exceeds drastically the ETactual rate. The triangular shaped area between the P and ETactual line represents the amount of meteoric water available for surface runoff and groundwater recharge.

Fig. 2. Topographical map of the Los Azufres geothermal field with the sampled cold (I Ciudad Hidalgo, II ‘‘Camp’’, III ‘‘Az-47’’, IV ‘‘Az-17’’, V ‘‘Az-6’’) and hot springs sites (1 San Alejo, 2 ‘‘Az-17’’, 3 ‘‘Leoncito’’, 4 Gallo, 5 Azufres, 6 Laguna Verde, 7 ‘‘Agrio’’, 8 Laguna Verde, 9 Currutaco, 10 Marı´taro), geothermal wells, as well as the water division between the northern and southern part of the field.


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Fig. 3. Seasonal distribution of the precipitation (P), potential evaporation (ETpot), and actual evapotranspiration (ETactual) in the Los Azufres geothermal area.

The quantitative hydrological balance for the extended study area resulted in a total annual input of 223106 m3 of precipitation within 183 km2, which corresponds to 1330 mm/m2/a (Fig. 4). 48% (637 mm) get lost by evapotranspiration processes, whereas 11106 m3 (62 mm) and 34106 m3 (186 mm) discharge from the study area in the form of surface runoffs during dry and wet season, respectively. The considerable amount of 446 mm (82106 m3) is available for groundwater recharge. Due to methodical errors in the semi-quantitative calculations and measurement techniques, a standard deviation of 206 mm must be accepted for the infiltration value. Due to the highly fractured and faulted structure of the volcanic formations and the low abundance of shallow aquifers in the Los Azufres zone (Birkle et al., 1997a), major fractions of meteoric water are considered to infiltrate deeper sections of the bedrock. 4.2. Stable and radioactive isotopic composition 4.2.1. Influence of andesitic and magmatic water Isotopic data from cold and hot springs and from production wells of the geothermal field of Los Azufres are given in Fig. 5. The composition of the deep fluids, recalculated under reservoir conditions, is shown for the initial (1980–1981) and intermediate (1985–1987) period of geothermal production (Nieva et al., 1983, 1987, respectively) (Table 1), as well as the recent composition of the geothermal fluids in 1998 (Barragań et al., 1999). The global meteoric water line, as well as the

Fig. 4. Quantitative hydrological balance for the extended study area (183 km2): column 1: total amount of precipitation (column number in mm); column 2: total amount of evapotranspiration, surface runoff during dry and wet period, and the amount available for infiltration processes.

hypothetical composition of primary magmatic water from hydrous basalts and subduction-related ‘‘andesitic’’ water (Taran et al., 1989, Giggenbach, 1992) are plotted as well.

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Fig. 5. 18O vs. D of geothermal brines, condensates, cold and hot springs of the Los Azufres geothermal field (location and names in Fig. 2, Hot springs: site number and water type in Table 2) compared to that of andesitic (Giggenbach, 1992) and magmatic water. Table 1 Recalculated reservoir values of 18O, D and Cl for geothermal fluids from different sample periods (Nieva et al., 19831, 19872)a Well

Date (d/m/y)

18O (%)

D (%)

Cl (mg/l)

Az-51 Az-51 Az-71 Az-81 Az-131 Az-191 Az-52 Az-92 Az-132 Az-16AD2 Az-182 Az-192 Az-262 Az-282 Az-322 Az-362

29/04/80 13/01/81 17/04/80 26/04/80 14/01/81 13/01/81 24/04/85 16/05/87 24/04/85 23/04/85 11/09/85 25/06/85 26/06/85 08/05/85 08/05/85 31/03/87

4.0 4.1 4.2 4.7 2.8 5.2 3.7 2.3 2.5 3.7 3.6 3.5 2.0 2.5 2.7 3.3

59 62 65 66 60 62 63 61 63 62 66 61 64 56 61 60

1,134 1,250 1,407 1,325 1,619 284 609 1,451 946 573 939 1,367 1,801 1,648 168 878

a

Superior 1 and 2 denote Refs.

The cold springs of the geothermal field are characterized by a typical meteoric composition (samples I, II and III), although two samples (IV and V) with elevated D and 18O values are affected by evaporation processes (Birkle, 1998).

The initial reservoir discharge from the geothermal field (Nieva et al., 1983) was characterized by the deviation of the 18O and D-values from the global meteoric water line towards more positive values. According to Giggenbach (1992), very large 18O shifts, such as observed in Los Azufres, cannot be explained exclusively by water–rock interaction. Thus, the positive isotopic trend of the initial fluid discharge, especially the increase in D in comparison to recent meteoric water composition, indicates the influence of additional processes. The intermediate position of the Los Azufres fluids between meteoric and ‘‘andesitic’’ water and/or magmatic water implies mixing between both components. Considering the latter process exclusively, a mixing ratio of about 30:70 between meteoric water and the fossil, magmatic component respectively would be postulated. Normally, mixing between two components should be reflected by a straight-line relationship between Cl and 2 H concentrations. Fig. 6 shows the Cl and D-values of the total discharge from different wells, sampled from 1980 to 1981 (Nieva et al., 1983) and 1985 to 1987 (Nieva et al., 1987) during the initial phase of geothermal production. In contrast to the expected straight-line relationship, a very heterogeneous distribution of the values is observed. Thus, mixing of two components is not the dominating formation process of the reservoir fluids. Considering the variation of the fluid composition with time, most wells show constant isotopic composition from 1980 to 1998 (Fig. 5). Beginning with average 18O and


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Fig. 6. Cl and D-correlation of the total discharge from different wells, sampled from 1980 to 1981 (Nieva et al., 1983) and 1985– 1987 (Nieva et al., 1987).

D values of 4.2 and 62.5% in 1980–1981 (Nieva et al., 1983), respectively, the reservoir fluids maintain a similar average 18O and D composition of 3.7 and 60.4% in 1998 (Barragań et al., 1999) (Fig. 5). A slight isotopic enrichment can be observed exclusively for production wells, which are adjacent to reinjection wells and affected by the reinjection of geothermal brines (Barragań et al., 1999). Similarly, the rise of the Cl content with time has been related to the reinjection of geothermal fluids into the reservoir (Sua´rez et al., 1995). 4.2.2. Vertical communication between reservoir and hot springs The hot springs and fumarole fluids from the geothermal field area show a very heterogeneous composition with strong variations from meteoric composition to extreme positive D and 18O values (Birkle et al., 1997b) (Fig. 5). Using chemical-isotopic data and results of 3H analysis, 4 different types of warm and hot springs (A-D) can be distinguished (Table 2, Fig. 5): . Type A: high mineralization, especially of typical geothermal tracer elements, such as Cl, B and F,

as well as very high D (24 to 34%) and 18O (3.4 to 5.6%) values, indicate the direct rise of geothermal liquid and vapour to the surface (Examples: Marı´taro, Laguna Verde spring). . Type B: missing 3H (0 T.U.), quite high D (24 to 39%) and 18O values (1.7 to 5.4%) as well as the combination of low Cl-concentrations (16– 29 mg/l) and enrichment in SO4 (640–660 mg/l) reflect the condensation close to the surface of rising vapour enriched in H2S as the principal process determining the Currutaco spring chemical composition and probably that of the upper course spring of the Agrio river (‘‘Agrio’’). . Type C: elevated 3H values (5.1–8.3 T.U.) and a low mineralization rate with SO4 as principal component indicate the heating up of a shallow aquifer (with a residence time of more than 10 a) by ascending vapour, which causes the deviation of the D (57 to 62%) and 18O (4.5 to 5.8%) values from the meteoric water line (examples: Gallo, Azufres) (Birkle et al., 1997b). . Type D: hot springs with a 18O vs. D composition close to the meteoric water line are fed by

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Table 2 Chemical and isotopic composition of hot springs in the Los Azufres area Tritium (T.U.)

5.6 3.4

24 34

n.m.a n.m.